In the medical field, reduction in wastes of processing solutions has been strongly desired from the viewpoints of environmental conservation and space saving in recent years. Therefore, new technologies regarding light-sensitive heat-developable photographic materials used for medical diagnosis and photographic technology, which can be efficiently exposed by means of a laser image setter or a laser imager and form clear black images having high resolution and high sharpness, have been requested. It is possible for such light-sensitive heat-developable photographic materials to eliminate the necessity of using solutions of processing chemicals and to supply simpler and environmentally friendly heat-developable processing systems for customers.
Though such requirements are also present in the field of common image-forming materials, images for medical use are required to have high qualities including excellent sharpness and graininess, because fine depiction is essential to them. Also, they have a feature that it is desirable to have a cold black tone from a viewpoint of ease of diagnosis. At present, various kinds of hard copy systems utilizing pigment or dye, for example, inkjet printers and electrophotography, are distributed as common image-forming systems, but they are not satisfactory as output system of images for medical use.
On the other hand, thermal image formation systems utilizing organic fatty acid silver salts are described, e.g., in U.S. Pat. Nos. 3,152,904 and 3,457,075, and B. Shely, Thermally Processed Silver Systems in Imaging Processes and Materials, Neblette's 8th Edition, compiled by Sturge, V. Walworth and A, Shepp, page 2 (1996).
In particular, a heat-developable photosensitive material ordinarily has a light-sensitive layer containing a binder matrix dispersed therein a catalytic amount of photocatalyst (e.g., silver halide), a reducing agent, a reducible silver salt (e.g., an organic fatty acid silver salt) and, if desired, a toning agent for controlling color tone of silver. After imagewise exposure, the heat-developable photosensitive material is heated at a high temperature (e.g., 80° C. or higher) to cause a redox reaction between the silver halide or reducible silver salt (acting as an oxidizing agent) and the reducing agent, thereby forming a black silver image. The redox reaction is accelerated by the catalytic action of a latent image of the silver halide formed upon the image exposure. Thus, the black silver image is formed in the exposed area. The heat-developable photosensitive materials are described in many documents including U.S. Pat. No. 2,910,377 and JP-B-43-4924 (the term “JP-B” as used herein means an “examined Japanese patent publication”). Also, Fuji Medical Dry Imager FM-DP L is available as a medical image formation system utilizing a heat-developable photosensitive material.
In the thermal image formation system utilizing an organic fatty acid silver salt, a heat-developable photosensitive material is produced by a method of using an organic solvent for coating, or a method of applying and drying a coating solution containing an aqueous dispersion of polymer fine particles as a main binder. Since the latter method does not include a step of recovering the solvent, it permits simplification of production facilities and is advantageous for mass production.
The mono-sheet heat-development image formation system utilizing an organic fatty acid silver salt does not include a fixing step. Accordingly, the system has a serious problem with image preservability after the development processing, particularly deterioration of print-out due to exposure to light.
Moreover, there is a large restriction in design of heat-developing machine because the heat-developable photosensitive material is prohibited from exposure to light until the heat-developed material has been completely cooled. Therefore, development of heat-developable photosensitive material capable of designing a simple and small-size heat-developing machine has been desired.
In JP-A-8-76317 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and JP-A-8-304952, methods of adding a large amount of mercapto compound in order to fix silver halide are described. However, such methods are disadvantageous since the large amount of additive is necessary.